High end PC running really slow after fresh windows install

Mattw96

Reputable
Mar 12, 2015
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4,510
So I got this pretty fancy PC with the following specs about a year ago:

GPU : GeForce GTX 760
CPU: Intel(R) Core(TM) 17-4770K CPU @ 3.50GHz (4 Cores)
Memory : 16.00 GB RAM (15.89 usable)
OS: Windows 8.1

I was away for about a month, and when I came back to the PC I did a fresh install of windows on it (totally cleared the C:// Drive). Now games that is used to run easily are lagging considerably. I've run them with task manager open, at the CPU is only using 10-20%, but there's still a lot of lag (about 5 FPS even on just the main menu). This has affected all the games i've tried to run.

Not being one to ask for help without trying to fix it myself first, here's what I've already done:

Updated all the drivers I can see (graphics, audio, motherboard, all the windows standard ones)
Run a full blown virus check with Bit Defender
Checked system temperature whilst running programs (Real-temp 3.7 says they're fine)
uninstalled & reinstalled games
Turned it off and on again

Any help would be hugely appreciated!!!